Among vehicle body side frames, frames are known that are formed by hot stamping so as to have a tensile strength exceeding 1400 MPa, with end portions of the side frames (hereinafter referred to as frame end portions) formed as soft portions. The tensile strength of the soft portions is suppressed to less than 1000 MPa.
The sheet thickness of the side frames is suppressed by raising the tensile strength of the side frames that exceeds 1400 MPa, and a reduction in weight of the side frames is achieved.
The frame end portions are formed as soft portions by reducing the tensile strength of the frame end portions to less than 1000 MPa. Thus when impact load has been applied to the frame end portions, along the longitudinal direction (axial direction) of the side frames, the impact load is absorbed by the frame end portions crushing in the longitudinal direction under the impact load (see, for example, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2012-528752).
However, in the vehicle body side frame of Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2012-528752, it is difficult to secure a large amount of crushing of the frame end portions due to the frame end portions being crushed along the longitudinal direction to absorb impact load. Thus the amount of absorption of the impact load by the frame end portions is reduced, and there accordingly remains room for improvement from this perspective.
Among vehicle body side frames, frames are also known that are formed by hot stamping with a tensile strength higher than 1400 MPa, with weak points formed so as to be different to each other on two side portions. The tensile strength of the weak points is reduced to a lower tensile strength of from 500 to 1000 MPa.
The sheet thickness of the side frames is reduced by raising the tensile strength of the side frames that exceeds 1400 MPa, and a reduction in weight of the side frames is achieved.
The tensile strength of the weak points is suppressed to a lower tensile strength in the range of from 500 to 1000 MPa. This thereby enables the mutually different weak points to be deformed when impact load has been applied to the side frames along the longitudinal direction, enabling the side frames to bend at the weak points. A deformation stroke of the side frames is secured by the side frames bending at the weak points, enabling the impact load to be absorbed (see, for example, International Publication No. 2015/001114).
In the side frames of International Publication No. 2015/001114, due to the bending at the weak points, portions facing the weak points are greatly stretched (hereinafter referred to as stretched portions). However, the stretched portions are formed with a tensile strength higher than 1400 MPa.
Thus, in a state in which the stretched portions have been greatly stretched, it is accordingly conceivable that cracks develop in the stretched portions, making it difficult to achieve favorable bending of the stretched portions. It is accordingly difficult for the stretched portions to bend and to secure an absorption amount of the impact load, leaving room for improvement from this perspective.